1. Technical Field
The present invention relates to a shift control device, and more particularly, to a shift control device of an automatic transmission having a parking mechanism for locking an output shaft.
2. Related Art
Generally, a driving force output from an engine is input to an automatic transmission via a torque converter, is converted by the automatic transmission, and then transmitted to a driving wheel via a power transmission system that includes a gear or a drive shaft. Therefore, for instance, in a state in which a shift position is in a D range (forward running position) or an R range (reverse running position), a brake pedal is depressed, and a vehicle is stopped, the drive shaft or the like constituting the power transmission system is twisted due to an engine torque input via the torque converter.
On the other hand, in an automatic transmission, when a P range (parking position) is selected, a parking gear fitted to the output shaft of the automatic transmission is locked (that is, the output shaft is fixed). Therefore, when the parking gear is locked in the state in which the drive shaft or the like constituting the power transmission system is twisted as described above, the twisted state of the drive shaft or the like constituting the power transmission system is maintained. Then, when shifting occurs from the P range to another range (for instance, the D range, or the like), by rotation of the parking gear, the drive shaft, or the like due to an accumulated torsional torque when the parking gear is unlocked and the maintained twist is eliminated, shock, vibration, or abnormal noise may be generated and cause a passenger to feel uncomfortable.
To solve the above problem, in Japanese Unexamined Patent Application Publication (JP-A) No. 2002-122236, a shift control device that prevents a torsional torque from remaining and prevents shock, abnormal noise, and the like when shifting from a P position to another position occurs is disclosed. More specifically, in the shift control device, when a shift lever is operated from an R position to the P position, a second clutch and a fourth brake in the automatic transmission are operated to a disengagement side, and after waiting for a predetermined time, the output shaft of the automatic transmission is locked by a parking lock mechanism. According to the shift control device, because the power transmission of the automatic transmission is disconnected prior to locking the output shaft, the output shaft is locked in a state in which there is no residual torsional torque in the power transmission system in the automatic transmission, and after that, shock is not generated even when shifting occurs from the P position to another position.
As described above, according to the technology described in JP-A No. 2002-122236, it is possible to prevent shock or abnormal noise accompanying release of torsional torque which is accumulated as torsional deformation. However, when the P range is selected and the parking gear fitted to the output shaft of the automatic transmission is locked in a state in which the drive shaft or the like constituting the power transmission system is twisted, for instance, until a clutch is disengaged and the parking gear and a parking pawl are fitted, the twist of the drive shaft or the like constituting the power transmission system is disengaged, the parking gear is rotated, and a rattling noise (abnormal noise) is generated due to the parking gear and the parking pawl coming into contact with each other in some cases.
However, in the shift control device described in JP-A No. 2002-122236, the rattling noise (abnormal noise) when the parking gear is locked is not considered as a problem. However, although the rattling noise (abnormal noise) is not considered as a problem, in the shift control device, when the shift lever is operated from the R position to the P position, the second clutch and the fourth brake in the automatic transmission are operated to the disengagement side, and after waiting for a predetermined time, the output shaft of the automatic transmission is locked by the parking lock mechanism. Thus, it is presumed that this can contribute to reduction of the rattling noise (abnormal noise) when the parking gear is locked.
The magnitude of the torsional torque (i.e., amount of twist) acting on the drive shaft or the like constituting the power transmission system varies depending on an operating state of a drive system (e.g., an operating state of an engine, etc.) when the parking gear is locked. Therefore, for instance, when the magnitude of the torsional torque (amount of twist) is large, depending on the length of the waiting time, there is a possibility that the rattling noise (abnormal noise) cannot be sufficiently reduced when the parking gear is locked. On the other hand, when the magnitude of the torsional torque (amount of twist) is small, the waiting time may be longer than necessary.